Electrophotographic reproduction techniques for making reproductions of graphic originals using a photoconductive medium are well known. One such technique is directed to the use of a foraminated device or screen comprising a conductive layer, a photoconductive layer and a superposed insulative layer capable of having stored thereon charge patterns corresponding to light and dark areas of a graphic original.
The layer charges are modified in accordance with an image to produce blocking and unblocking fields controlling the apertures in the screen in accordance with the image to be reproduced. The conductive screen layer is maintained at a potential, usually during charging and printing, and a propulsion field is provided for directing charged printing particles towards the screen.
The charged printing particles pass through the screen where the apertures are not blocked by the blocking fields and also pass through apertures which are partially blocked, but in fewer numbers. The charge pattern modulates the flow of the printing particles through the screen to a print receiving medium, via an airgap, for subsequent development by conventional techniques.
In another aperture-controlled electrostatic printing system there is provided a photoconductive screen as described above including means for deploying oppositely poled electrostatic charges across the screen, and image projecting means for modulating the charge on the screen in accordance with a light image received thereon. However, this system includes a corona source for projecting gas ions through the unblocked apertures in the screen, and through the partially blocked apertures but in fewer numbers, to an image receiving medium for subsequent development by any conventional means.
Although electrostatic screen printing techniques are known in the art, lacking in the art is any showing of an arrangement for incorporating the screen technology into practical machine configurations suitable, for example, for copier-duplicator or reader-printer machines. Thus, because of the requirement of mechanical integrity of the photoconductive screen it does not lend itself to drum or cylinder configuration of rigid construction critical for screen drum printing. Therefore, in the known devices the photoconductive screen is normally in the form of an endless belt trained about a plurality of rollers for supporting the screen.
Such an arrangement requires that the photoconductive screen be driven intermittently to permit imaging the copy sheet at a printing station or, if the screen is to be continuously in motion, it is necessary to synchronize the motion of the screen with the travel of the copy sheets past the printing station. In either case, the known constructions result in fairly complex and expensive devices which are not suited to or feasibile for incorporation into a compact, high-speed copier of practical machine configuration.